1. Field of the Invention
The present invention relates to a medical image processing apparatus, a medical image processing method, and a medical image processing program for processing medical images obtained by dual energy imaging.
2. Description of the Related Art
Along with advancement of medical devices (such as multi-detector CT apparatuses) in recent years, high-quality three-dimensional images are beginning to be used for imaging diagnosis. A three-dimensional image is formed by a number of two-dimensional tomographic images and thus has a large amount of information. Therefore, it may take time for a doctor to find and diagnose a desired observation site. To address this problem, various techniques have been proposed to enhance visibility of the entire structure, and further a lesion contained in the structure, by recognizing a structure of interest, and generating, from a three-dimensional image containing the structure of interest, a three-dimensional image of the structure of interest with using, for example, a method such as maximum intensity projection (MIP) and minimum intensity projection (MinIP) to achieve MIP display, to achieve volume rendering (VR) display of the three-dimensional image, or to achieve CPR (Curved Planer Reconstruction) display (i.e., pseudo three-dimensional display).
On the other hand, as a technique to achieve substance separation in tomographic images of a subject, a technique using dual energy imaging has been known. This technique uses characteristics of substances that X-ray absorptions of substances vary depending not only on the type of each substance but also on the X-ray energy. In this technique, a subject is imaged with a CT apparatus using two types of X-rays having different energy distributions, and pixel values (CT values) of corresponding pixels between the thus obtained two types of tomographic images are compared with each other to separate substances contained in the tomographic images. For example, a technique proposed in Japanese Unexamined Patent Publication No. 2009-178493 (which will hereinafter be referred to as Patent Document 1) involves obtaining high-energy tomographic images and low-energy tomographic images by dual energy imaging with switching the voltage of the X-ray tube between a high voltage and a low voltage, and separating substances contained in the images based on a ratio between corresponding pixel values of the high-energy tomographic image and the low-energy tomographic image.
It is known that, by carrying out addition or subtraction between the high-energy tomographic image and the low-energy tomographic image obtained by dual energy imaging, a composite image where a bone and a calcified lesion are enhanced, or a composite image where a soft tissue is enhanced can be obtained. The high-energy tomographic image obtained by dual energy imaging has low contrast and low noise. In contrast, the low-energy tomographic image has high contrast and high noise. Therefore, when a composite image is obtained, the user, who is a doctor to read the composite image, generates the composite image with controlling the composition ratio for addition of the high-energy tomographic image and the low-energy tomographic image so that image quality that facilitates recognition of a desired tissue, such as a tumor, is achieved.
Further, a technique involving displaying a plurality of composite images obtained by combining the high-energy tomographic image and the low-energy tomographic image at different composition ratios and allowing the user to select one of the composite images having desired image quality has been proposed (see S. Kitano, “Dual Energy Imaging—abdominal region—”, Inner Vision, supplement of the November issue, 2009, which will hereinafter be referred to as Non-patent Document 1). According to the technique disclosed in Non-patent Document 1, the user can select one of the composite images having desired image quality with comparing the composite images.
However, the technique of generating a composite image with controlling the composition ratio of the high-energy tomographic image and the low-energy tomographic image requires the user to change the composition ratio and check the generated composite image by trial and error, and thus imposes a large burden on the user. Further, the technique disclosed in Non-patent Document 1 generates a plurality of composite images, and thus requires the user to read all the generated composite images to select one having desired image quality. This also imposes a large burden on the user.